As a technique of this type, heretofore, a rotation sensor has been widely used in various fields. For vehicle engines, a crank angle sensor which is one of rotation sensors is used to detect rotation speed and rotation phase of an engine. Such type of crank angle sensor is disclosed in JP2001-041092A.
A crank angle sensor using a magnetic pickup is known as a typical example of crank angle sensors. This sensor is commonly used in such a manner that the magnetic pickup consisting of a magnet and a coil is placed to face a gear-like magnetic member provided on a rotary shaft, and the distance of the gap or clearance between the pickup and the magnetic member is changed to thereby output a voltage waveform from the magnetic pickup. However, this system is problematic in that the magnetic pickup has a limitation in sharpening magnetic flux at a leading end of the magnetic pickup and also the gear-like magnetic member has a limitation increasing of the number of gear teeth, resulting in a limit on angle resolution.
Further, an optical rotary encoder is generally known as another system for detecting rotation. One example thereof is disclosed in JP6(1994)-095798A. However, the rotary encoder, which employs light, is apt to be affected by adhesion of contaminants. If a slit is made narrower to increase the resolution, the slit is liable to be clogged with contaminants. It is therefore difficult to use such rotary encoder in an adverse environment that is likely exposed to oil, dust, and others.
Instead of the above optical type, an electromagnetic induction type rotary encoder configured to utilize changes in magnetic field is conceivable to avoid the above problems with contaminants. One example thereof is disclosed in JP9(1997)-170934A. This electromagnetic induction type rotary encoder includes a magnet fixed to a rotary body and a plurality of coil patterns placed to face the magnet and arranged to detect the passage of the magnet associated with rotation of the rotary body. Those coil patterns are placed with their phases electrically displaced within a coil pattern region on a printed circuit board and with electric phase displacement.
However, in the electromagnetic induction type rotary encoder disclosed in JP9(1997)-170934A, the magnetic field around a detection coil is changed by movement of the magnet associated with the rotation of the rotary body, thereby generating induced currents in a coil pattern (one turn coil) serving as a detection coil. To obtain sufficient output, however, the rotary body has to be rotated at a higher rotation speed than a certain level. While the rotary body is rotated at a low speed, the angle could not be detected. It is conceived to increase the number of turns of the detection coil or increase the size of the magnet to obtain higher outputs and wide detectable rotation speed range. However, this may cause a problem with an increase in size of the rotational position sensor.
On the other hand, as a method not increasing the number of winding turns of the rotational position sensor, JP2000-292205A discloses the use of a high frequency excitation signal for a resolver to be used as a rotational position sensor.